1. Field of the Invention
The invention relates in general to a control keyboard of an electric or electronic device, of the type comprising a grid or an apertured plate formed with a multiplicity of holes in which are engaged keys each adapted for controlling one or several electric contacts when they are depressed.
2. Description of the Prior Art
In the present technology, the keys are generally returned to their rest position by springs. The use of springs leads to many disadvantages: loss of rigidity of the spring, fatigue phenomenon, resonance frequency, bad behaviour to vibrations and accelerations, sensitivity to corrosion, rebound phenomenon when establishing the electric contacts, etc.
It has already been proposed to remove most of said disadvantages by replacing the return springs with magnetic return means which are in part carried by the keys themselves, and in part carried or formed by the aforementioned grid or apertured plate.
This new technique, while presenting many advantages relative to the traditional art, has nevertheless a disadvantage in that the contacts, which are adapted to be acted upon by the keys when the latter are depressed, are carried by an independent plate, generally a printed circuit plate, which is placed under the grid or apertured plate. Thus, a keyboard is composed of three kinds of elements: the keys, the grid or apertured plate, and the printed circuit plate carrying the contact or switching elements associated with the keys. The mounting of such a keyboard has to be relatively accurate since the printed circuit plate carrying the contact elements has to be disposed under the grid at a predetermined distance from the latter.
A further disadvantage of the known prior art techniques is that the duration of the electrical signal corresponding to the closing of a contact obtained by depressing a key is very short relative to the total operation time of the key. However, for some technical reasons, and notably the electronic processing of said signals, it is often required that the duration of the signal thus established be at least equal to 60% of the operation time of the key.